Understanding D/d Ratio
Passing a sling—synthetic or other—around a load can provide support by creating a cradle, but you must make sure that it is securely attached. Therefore, you must understand D/d ratio.
The all-important D/d ratio is the ratio of the diameter (D) around the object which the sling is bent, divided by the overall diameter (d) of the sling being used.
First, let’s look at when this equation is applied. In the absence of suitable lifting points, the main options are to attach slings by passing them through or around the load. Passing through a suitable aperture has the advantage of making the load captive but you need to be careful that the material around the aperture is strong enough to take the whole load. Other aspects of the load to consider are whether it is flexible, in one piece or needs holding together, is vulnerable to local damage, or must be manipulated in the air. It is worth stressing the need to consider at the planning stage the position and security of the method of attachment through all the stages of manipulation. A typical example of a load that needs holding together is a bundle of long slender items such as reinforcing rods.
Here in Australia, the D/d ratio impacts sling capacity primarily when using synthetic slings, versus wire rope slings and chain slings, which might be more relevant in other parts of the world. When such a sling is bent around another object there is a loss of capacity. It is also worthwhile adding that our safety factors here for synthetics are minimum 7:1 and not 5:1, as in other parts of the world.
Here’s a three-point example:
- 1. We’re picking up a 300mm diameter piece of pipe.
- 2. We’re using a 10mm chain sling to rig it.
- 3. 300mm divided by 10mm = a D-d ratio of 30.
All slings lose capacity when bent
While it’s true that we most commonly talk about D/d ratio when using synthetic roundslings / flat webbing slings, all slings lose capacity when bent too much. We will look at the different types of slings and how D/d ratio affects these below.
Chain is a durable and versatile option; it is flexible, and able to wrap around the load. There is no derating on the sling if the D/d ratio is larger than two times diameter on a 90-degree edge, whereas if it is smaller than one times diameter there is a deration of half the rated capacity of the sling working load, as mentioned in both AS 3775 and AS 2321.
Note that packing is often required between the sling and the load to prevent chain links being bent over a corner of the load or the chain damaging the surface finish of the load.
Wire Rope Slings
Wire rope offers an economical alternative to chain for many applications and has advantages of its own. It is easier to feed under a load as it can often be pushed through. Note that slings made from galvanised rope and thimbles are more tolerant of marine or similar environments.
The condition of the wire rope (or chain) must always be checked, keeping in mind that it is likely to be worst at those points where it engages most frequently with the rope sheaves or load chain wheel.
The rule of thumb for wire rope is ‘the bigger the better’ when it comes to D/d ratio—and the avoidance of reverse bending, tight sheaves, damaged sheaves, fleet angles, etc. all impact wire rope life. It also of course depends on the construction, lay, and tensile of the wire rope. Many of the newer ropes are more tolerant than the previous 6 by 25 ropes (or six strands of 25 wires) but the same principle of ‘the bigger the better’ still applies.
Note that Standards Australia does specify minimum recommended D/d ratios for various rope constructions, while manufacturer recommendations are referred to with referenced to newer constructions not covered in AS documents.
The term ‘synthetic’ encompasses slings made from fibre rope and common flat webbing slings, endless round slings, and now even synthetic chain. Synthetic slings have come a long way in the last few years with newer generation high-modulus polyethylene (HMPE) fibres providing very high capacity but are physically very small and extremely lightweight.
Synthetic chains are now being manufactured as well to provide further options in the synthetic sling category. Synthetics are soft compared to chain and wire rope and are therefore less likely to damage the surface of a load, but they can easily be cut if loaded over a sharp edge or corner without adequate packing or sling protection (see insert). Synthetic slings were previously limited by lack of length adjustment, but this issue has now been solved with the introduction of synthetic chains.
D/d ratios for common polyester and HMPE sling products should always be considered and should be checked carefully with the manufacturer or supplier. Sling saver shackles are advised when using large capacity HMPE slings as they increase the D/d ratio significantly.
PROTECTING THE LOAD
Many loads are vulnerable to local damage and need protecting.
Generally, synthetic slings are less likely to cause local damage than wire rope or chain slings. Round slings flatten under load thus reducing the pressure. A greater spread can be achieved with webbing slings, but particular care is required to ensure they are uniformly loaded across their width. Sling protective sleeves are a good option for synthetic slings to be used at the lift points around shackles etc. These sleeves assist with D/d ratio and prolong the life of the slings as the sleeves are usually a Velcro on option and can be moved along the sling easily so lift points can be shared along the length of a round slings.